I am trying to inherit "account.bank.statement.line" model.  For this i am writing a below code. it's installing perfectly but fields not adding to table

.py file

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from openerp.osv import fields, osv

class account_bank_statement_line_extension(osv.osv):
    _inherit = 'account.bank.statement.line'
    _column={
    'payment_reference':fields.char('payment_reference',size=50),
    }
    
account_bank_statement_line_extension()

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__openerp__.py

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{
'name': "Account Cashflow Extension",
'version': "1.0",
'author': "Samba",
'category': "Tools",
'description':"""
This module for to add payment_reference field to account_bank_statment_line
""",
'depends': ['account'],
'data': [

],
'demo': [],
'installable': True,
}

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I don't have any view files. column "payment_reference" not adding to table "account_bank_statement_line".
To check weather column added or not, i am using pgadmin3. I have restarted pgadmin3 also.